The invention resides in a method for pressure regulation of an accumulator of a fuel injection system of an internal combustion engine wherein an actual rail pressure is determined via a filter from the measured rail pressure, a control deviation is calculated from the desired rail pressure and the actual rail pressure and a volume flow is determined via a high pressure controller as a control value depending on the control deviation.
In an accumulator fuel injection system, a high pressure fuel pump pumps fuel from a fuel tank into an accumulator rail. The fuel admission cross-section of the fuel supply line to the high pressure pump is controlled by a variable suction throttle. From the accumulator rail, the fuel is supplied to the combustion chambers of the internal combustion engine by way of fuel injectors mounted into the cylinder head of the engine. Since the quality of combustion depends greatly on the pressure level in the rail, the fuel pressure level in the rail is controlled via a high pressure control circuit. The high-pressure control circuit comprises a high pressure controller, the suction throttle with the high pressure pump and the rail as control path as well as a filter in a back-coupling branch. Typically, the high pressure controller is in the form of a PID controller or a PIDT1 controller, that is, it comprises at least one proportional part (P-part), an integral part (I part) and a differential part (D-part). In this high pressure control path, the pressure level in the rail corresponds to the control value. The pressure values measured in the rail are converted via the filter into an actual rail pressure and compared with a desired rail pressure. The control deviation obtained therefrom is converted by way of the high pressure controller into a control signal for the suction throttle. The control signal corresponds, for example to a volume flow with the unit liter/minute. The high-pressure control path described here-above is known from the non-pre-published German patent application DE 103 30 466.5.
As protection from an excessively high pressure level, the rail is provided with a passive pressure limiting valve. When the pressure level becomes excessive, the pressure limiting valve opens whereby fuel is conducted from the rail back to the tank.
However, in practice, the following problem can occur: With a sudden load drop, the engine speed increases. An increase in the engine speed results, with a constant desired speed, in an increase in the speed control deviation. As a reaction, the speed controller will reduce the fuel injection amount as control value. A smaller fuel injection amount again causes a reduction of the fuel amount being removed from the rail so that the pressure level in the rail increases rapidly. An aggravating condition is that the pumping volume of the high pressure pump is speed-dependent: An increase in the engine speed results in an increased pumping volume and an additional pressure increase in the rail. Since the high pressure control has a relatively large reaction time, the rail pressure may increase to such a degree that the pressure limiting valve opens, for example, at 1950 bar. Then the rail pressure drops for example to a value of 800 bar. At this pressure level, an equilibrium state of the fuel amount pumped and the fuel returned to the tank is established. This means that, in spite of the opened pressure limiting valve, the rail pressure does not drop any further. The pressure limiting valve closes only, when the engine speed is reduced. Therefore, the unexpected opening of the pressure limiting valve as a result of a sudden load decrease is problematic.
It is the principal object of the present invention to improve the safety of the pressure control of an accumulator fuel injection system.